SQL Injection

Many web developers are unaware of how SQL queries can be tampered with,
and assume that an SQL query is a trusted command. It means that SQL
queries are able to circumvent access controls, thereby bypassing standard
authentication and authorization checks, and sometimes SQL queries even
may allow access to host operating system level commands.

Direct SQL Command Injection is a technique where an attacker creates or
alters existing SQL commands to expose hidden data, or to override valuable
ones, or even to execute dangerous system level commands on the database
host. This is accomplished by the application taking user input and
combining it with static parameters to build an SQL query. The following
examples are based on true stories, unfortunately.

Owing to the lack of input validation and connecting to the database on
behalf of a superuser or the one who can create users, the attacker
may create a superuser in your database.

Example #1
Splitting the result set into pages ... and making superusers
(PostgreSQL)

Normal users click on the 'next', 'prev' links where the $offset
is encoded into the URL. The script expects that the incoming
$offset is a decimal number. However, what if someone tries to
break in by appending a urlencode()'d form of the
following to the URL

If it happened, then the script would present a superuser access to him.
Note that 0; is to supply a valid offset to the
original query and to terminate it.

Note:

It is common technique to force the SQL parser to ignore the rest of the
query written by the developer with -- which is the
comment sign in SQL.

A feasible way to gain passwords is to circumvent your search result pages.
The only thing the attacker needs to do is to see if there are any submitted variables
used in SQL statements which are not handled properly. These filters can be set
commonly in a preceding form to customize WHERE, ORDER BY,
LIMIT and OFFSET clauses in SELECT
statements. If your database supports the UNION construct,
the attacker may try to append an entire query to the original one to list
passwords from an arbitrary table. Using encrypted password fields is
strongly encouraged.

If this query (playing with the ' and
--) were assigned to one of the variables used in
$query, the query beast awakened.

SQL UPDATE's are also susceptible to attack. These queries are
also threatened by chopping and appending an entirely new query to it. But
the attacker might fiddle with the SET clause. In this
case some schema information must be possessed to manipulate the query
successfully. This can be acquired by examining the form variable names, or
just simply brute forcing. There are not so many naming conventions for
fields storing passwords or usernames.

But a malicious user sumbits the value
' or uid like'%admin% to $uid to
change the admin's password, or simply sets $pwd to
hehehe', trusted=100, admin='yes to gain more
privileges. Then, the query will be twisted:

<?php

// $uid: ' or uid like '%admin%$query = "UPDATE usertable SET pwd='...' WHERE uid='' or uid like '%admin%';";

MSSQL Server executes the SQL statements in the batch including a command
to add a new user to the local accounts database. If this application
were running as sa and the MSSQLSERVER service is
running with sufficient privileges, the attacker would now have an
account with which to access this machine.

Note:

Some of the examples above is tied to a specific database server. This
does not mean that a similar attack is impossible against other products.
Your database server may be similarly vulnerable in another manner.

Avoidance Techniques

While it remains obvious that an attacker must possess at least some
knowledge of the database architecture in order to conduct a successful
attack, obtaining this information is often very simple. For example,
if the database is part of an open source or other publicly-available
software package with a default installation, this information is
completely open and available. This information may also be divulged
by closed-source code - even if it's encoded, obfuscated, or compiled -
and even by your very own code through the display of error messages.
Other methods include the user of common table and column names. For
example, a login form that uses a 'users' table with column names
'id', 'username', and 'password'.

These attacks are mainly based on exploiting the code not being written
with security in mind. Never trust any kind of input, especially that
which comes from the client side, even though it comes from a select box,
a hidden input field or a cookie. The first example shows that such a
blameless query can cause disasters.

Never connect to the database as a superuser or as the database owner.
Use always customized users with very limited privileges.

Use prepared statements with bound variables. They are provided
by PDO,
by MySQLi
and by other libraries.

If the application waits for numerical input, consider verifying data
with ctype_digit(), or silently change its type
using settype(), or use its numeric representation
by sprintf().

Example #5 A more secure way to compose a query for paging

<?php

settype($offset, 'integer');$query = "SELECT id, name FROM products ORDER BY name LIMIT 20 OFFSET $offset;";

// please note %d in the format string, using %s would be meaningless$query = sprintf("SELECT id, name FROM products ORDER BY name LIMIT 20 OFFSET %d;",$offset);

?>

If the database layer doesn't support binding variables then
quote each non numeric user supplied value that is passed to the
database with the database-specific string escape function (e.g.
mysql_real_escape_string(),
sqlite_escape_string(), etc.).
Generic functions like addslashes() are useful only
in a very specific environment (e.g. MySQL in a single-byte character
set with disabled NO_BACKSLASH_ESCAPES) so it is
better to avoid them.

You may use stored procedures and previously defined cursors to abstract
data access so that users do not directly access tables or views, but
this solution has another impacts.

Besides these, you benefit from logging queries either within your script
or by the database itself, if it supports logging. Obviously, the logging is unable
to prevent any harmful attempt, but it can be helpful to trace back which
application has been circumvented. The log is not useful by itself, but
through the information it contains. More detail is generally better than less.